AthelR said:

My plant is showing signs of magnesium deficiency... I’ve treated it with Epsom salts (pure MgSO4) twice a week since. It is suddenly getting worse. Am I using too little.

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That should be enough.

AthelR said:

Advice on dosage would be welcome.

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It's typically added to water at a dose of 1 tbsp / gallon.

AthelR said:

Could it be something else, not Mg deficiency? I’m worried it will effect Cola development which should start any time now.

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It could be. The leaf in the picture doesn't quite look like Mg deficiency to me, but it's a tough call. Otherwise, I suspect soil acidity could be the problem. The sulfur in MgSO4 can increase the acidity of soil, which would make the symptoms worse if acidity is in fact the problem. A soil pH test would be helpful for diagnosis, or you could top dress a few tablespoons of calcitic lime to raise the pH to see if it helps. Dolomite lime has Mg in it, which would be a better option than Epsom salts for acidic soil. Stop the Epson salts for a while, as well.

AthelR said:

I’m worried it will effect Cola development which should start any time now.

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From what I can see of the plant in the picture, it looks okay. Flowering does require a different nutrient mix, though, so deficiencies tend to show when the plant changes its priority to reproduction. That means less nitrogen and more phosphorus and potassium. Your plant doesn't quite look like it's at that stage yet.

A classic sign your soil has gotten out of whack is the plants look even worse after a feeding or a watering.

Uneaten nutrients can accumulate in the soil and present themselves as mineral salts. Those salts begin to slowly turn the soil acidic. As it does, the plant is unable to uptake nutrients that require a higher pH, usually calcium and magnesium which become unavailable at about 5.8 pH. So now you're looking at your plant and thinking it needs calcium so you feed it, plant doesn't eat it and instead contributes more to the mineral salts making it even more acidic and looking worse.

So this is the point you should stop putting anything else on the plant and run a slurry or a runoff test and see what the ppm, EC and pH look like. You don't even need an EC. You can use a pH pen and if it's clocking 5.9 or lower and that's regular soil, the salt has most likely accumulated and is driving the pH down. If this is what's going on, what you'll need is simply more volume of water going into the medium when you water until you can get better numbers coming out. Recovery time is usually about 2-3 weeks and will look worse at first when fixing before it finally levels off.

My story about soil acidity:

I have been reusing soil for several years. I knew I would need to amend it to keep it fertile, but that involved learning a few things. With each new grow, my plants were growing slower and smaller. Eventually, after doing some research, I concluded the pH was low. People say that pH isn't a problem with soil, but I found that's not true. To correct my soil's pH, I first used calcitic lime and then later used dolomite lime. The improvement was amazing and convinced me to buy a soil pH meter. Before I bought the meter, I only guessed how much lime to use, so I got lucky. Every pot was between 6.4 and 7.0. It continued to drop slightly, so I amended slightly more lime.

Soil does seem to become acidic and there are several causes. Salts (which are the result of an acid combining with a base) can be acidic, neutral, or basic. So, the effect on the soil can vary depending on the ingredients used. The trend seems to be toward lowering pH, though. There are other acidifiers, as well. Decomposing organic matter can also lower pH. (Compost, on the other hand, is neutral.) The release of sulfur from sulfates such as Epsom salt and gypsum can also lower pH due to the formation of sulfuric acid. (Based on my research so far. New learning could change that.)

Ninjadogma said:

As it does, the plant is unable to uptake nutrients that require a higher pH

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This is quite right. Also, a low pH can cause excess micronutrient toxicity and a pH above 7.0 can reduce macronutrient availability.

Thus, a soil pH tester can be a valuable tool for the organic soil grower. I've seen differing opinions about the best pH for soil, but most of what I've read agrees that a pH within a range between 6.3 to 7.0 is what's most important. I've used 6.4 as my lower limit and the plants are pleased.

Note that the pH story is different for growers who use synthetic nutrients in soil. These nutrients require a lower pH than organic nutrients. It can be confusing, so it can be helpful to specify the type of nutrients used.